This invention relates generally to amplifiers and more particularly to temperature compensated amplifiers.
As is known in the art, amplifiers are used in a wide variety of applications. For example, modern wireless communication products such as cellular telephones, satellite receivers and pagers require amplifiers to boost very weak signals that enter from the antenna to levels useable in subsequent receiver processing circuits. Amplifiers are used in transmitters as well, raising the power level emanating from message modulation circuits to that suitable for long range transmission.
As is also known in the art, one problem with many amplifiers is that their electrical characteristics change, and in particular their gain level, changes with ambient temperature. In many applications, some sort of temperature compensation is required to allow sensible operation without saturation. Several temperature compensation techniques have been suggested. One technique used is to change the operating current of the transistors used in the amplifier in accordance with the temperature. This technique, while inexpensive, is not very desirable when linearity and noise figure are important because, by the time the operating current is changed enough to change the gain the noise figure and linearity have been compromised.
Another more complex technique is to install a variable attenuator in the amplifier chain of plurality of serially coupled amplifier stages and change its attenuation in accordance with temperature. While this technique is more costly than the above described technique, better results can be obtained since the amplifier stages maintain a more constant noise figure and saturation level. Further, more than one attenuator is sometimes needed to keep the operating power levels in the entire amplifier chain within reasonable limits over the operating temperature range. Besides the cost of the additional attenuators, more amplifier stages must be added to overcome the insertion loss of the attenuators. Further, tracking problems arise when an amplifier stage yields more or less gain than predicted due to manufacturing variations, which then means an attenuator in the same amplifier chain is set to the wrong level for proper system operation.
A still more complex technique, used in expensive systems, is to use digitally programmed attenuators. The settings for these attenuators are individually determined at the time of manufacture by means of measuring the system performance over temperature and determining what settings are necessary for proper system operation. Sometimes even the frequency of operation is needed as well. All this information is stored in a digital programmable memory (PROM) and later used by a digital processor with its inputs of ambient temperature and frequency of operation and other system variables to determine the proper attenuator setting for system operation.